The present invention relates to one-part foamable polyurethane compositions and methods for their use.
Polyurethane foams are used in a wide variety of applications. The most typical method for making these foams involves bringing together a polyisocyanate component and a polyol component, and permitting them to react, foam and cure. Methods of this type require dispensing equipment, which tends to be expensive and requires a significant amount of space. These methods can release volatile materials that are present in the foam formulation, such as monomeric isocyanates, amine and tin catalysts, amine curing agents, and the like.
These methods are entirely suitable for making large foam buns, such as in a slabstock foam process. They are also suitable for making large quantities of molded foams of various types, such as flexible or semi-flexible molded foam, or rigid insulation foam as is commonly used in refrigeration devices and coolers. But these methods are less cost effective for some applications, notably those in which small quantities of foam are produced, in which the amount of foam that is needed is small, or in which it is difficult to provide good ventilation. Applications of this type include various automotive foam applications. Polyurethane foam is used in automotive applications to provide sound and vibration dampening, to seal joints or cavities to prevent water and oil penetration, provide energy absorption in case of a crash, or for other reasons. In many instances, the foam must be inserted into a small cavity on the frame or body of the vehicle (or some part or subassembly thereof). The volume of foam that is needed is usually small in each instance, and the total amount of foam that is needed per vehicle also tends to be small. The foam formulation often must be applied at close quarters, which can make ventilation difficult. Because of these difficulties, manufacturers often opt to use pre-formed foam, which must be manually inserted into place. It would be more desirable in many cases to apply a material that can foam in place, if costs and worker exposure issues could be addressed adequately.
One approach that could avoid the need for expensive equipment would be to provide a one-component foam formulation. One-component polyurethane formulations are known and used in some applications. For example, U.S. Pat. No. 5,817,860 describes a moisture-curable polyurethane that is useful in various sealant applications. U.S. Pat. No. 5,075,407 describes a one-component foamable polyurethane composition.
The difficulty in formulating a one-component polyurethane foam is to provide enough blowing agent to get good expansion, and at the same time create a composition that is storage-stable and cures only under defined circumstances. Most polyurethane foam is blown using water or a physical blowing agent. Water is very reactive with isocyanate groups, and so can be formulated into a one-component foam composition with great difficulty, if at all. The physical blowing agents tend to be too volatile to formulate into one-component compositions. Compositions containing physical blowing agents would need to be packaged under pressure, maintained under controlled temperature conditions, or both, in order to prevent the blowing agent from volatilizing prematurely.
Thus, attempts to make one-component polyurethane formulations have tended to follow one of two approaches. One approach is to rely on atmospheric moisture to provide the cure. Such an approach is described in U.S. Pat. No. 5,817,860. As atmospheric moisture reacts with isocyanate groups in the formulation, carbon dioxide is generated and the formulation expands somewhat as it cures. However, the amount of expansion is not great, and a low density foam is not obtained.
A second approach involves “tying up” water so it is not available for reaction until specific conditions are met, notably an elevated temperature. This is the approach described in U.S. Pat. No. 5,075,407. Water is provided in the formulation, in the form of waters of hydration of a salt. The formulation is stable until heated to a temperature high enough that at least some of the waters of hydration are released.
Once released, the Water is available to react with the polyisocyanate, and expand and cure the foam. However, U.S. Pat. No. 5,075,407 reports volume expansions of up to only about 433% with this approach. This corresponds to a foam density of about 15 pounds/cubic foot (240 kg/m3), which is much higher than is either necessary or desired. It therefore would be desirable to provide a method for producing low density polyurethane foam from a one-component foamable composition.